(1) Field of the Invention
The present invention relates to a toner for electrophotography for visualizing the latent image formed on the image support provided for an image forming apparatus using the electrophotographic technology, such as a copier, a printer and a facsimile machine.
(2) Description of the Prior Art
In an image forming apparatus using the electrophotographic technology, such as a copier, printer, facsimile machine or the like, a static latent image is formed on the photoreceptor surface as a static latent image support. In order to visualize this latent image, the apparatus has a developing unit which supplies the developer, e.g., toner, etc., as a coloring agent, to the photoreceptor so as to make the toner adhere thereto.
The static latent image formed on the photoreceptor is developed through the aforementioned developing unit, and the thus developed, toner image is transferred to a sheet of paper as printing paper. After the transfer station, part of the toner, which could not be completely transferred, will be left over on the aforementioned photoreceptor surface. This unused, leftover toner needs to be removed from the photoreceptor surface in order to perform subsequent image forming. For this purpose, a cleaning unit for removal of the toner left over on the photoreceptor surface is provided after the transfer station. The unnecessary toner removed by the cleaning unit is collected by the collecting portion inside the cleaning unit.
Since the toner image transferred to a sheet of paper remains unfixed, it is subjected to fixing to the sheet. This fixing process usually uses heat pressing. For example, a fixing unit comprises a heat roller disposed on the side in contact with the toner image and heated at a temperature allowing for the fusion of the toner, a pressing roller which is urged by an appropriate pressing to bring the sheet with a toner image thereon into close contact with the heat roller. This heat-press type fixing unit thus configured has been widely used because of its improved heat efficiency and high fixing efficiency.
However, this fixing process provides an increased heat efficiency, but suffers from the problem of setoff in that the heat roller surface makes contact with the fusing toner and hence the toner transfers to the heat roller surface, and in turn is transferred to a next sheet. In order to eliminate this problem, a cleaner is provided to clean the heat roller surface after the fixing process. Even when this kind of cleaner is used, there are cases where the toner having stuck can not be removed completely. Also to deal with this, a means for preventing the toner from sticking to the heat roller is provided.
One example of this is the application or coating of an anti-setoff agent onto the heat roller. For example, a separation agent such as silicone oil presenting a good separation performance with respect to the toner is put over the heat roller so that the toner supported on the sheet will not adhere to the heat roller during the fixing process.
As another example, there is a preparation of toner which itself is designed not to adhere to the heat roller. For example, in the process of producing the toner, when the starting materials for the toner are mixed, a separation agent such as low molecular weight polypropylene wax etc., is added so as to be dispersed during fusing and kneading. This method prevents the toner supported by the sheet from adhering to the heat roller.
A toner having wax as a separation agent contained therein and its production method in order to prevent the setoff phenomenon are disclosed, for example, in Japanese Patent 2,583,754. This toner contains low-molecular waxes so as to provide a separation performance. This improves the anti-setoff performance. The waxes here include polyolefin, polypropylene, polyethylene etc.
As stated above, by the selection of waxes to be contained in the toner, it is possible to prevent its adherence to the heat roller, and hence this type of toner is effective in eliminating the setoff phenomenon, that is, the adherence of the toner to the heat roller etc., during fixing.
On the other hand, if a lot of wax is used in order to solve the problem of the dispersion performance of the wax and in order to provide high enough separation performance, the wax adheres to the photoreceptor upon development, causing a new problem, that is, occurrence of image defects. Specifically, if wax adheres to the photoreceptor, it cannot be removed by the cleaner, and will adhere to the photoreceptor surface in film-like forms, which will be called `filming phenomenon`.
This phenomenon degrades the photoreceptor characteristics, causing the increase and/or decrease of the image density, fogging, and other defects, which significantly influences the image quality. This problem does not only stem from the toner, but is also considered to be attributed to the elevation of temperature within the developing unit with the development of the performance of the image forming apparatus into high speed one.
For the above reasons, a preparation of toner is needed which, without using a lot of wax, can eliminate the setoff phenomenon during fixing whilst eliminating the filming over the photoreceptor, to thereby provide stable image quality.
Further there is a concern in that deterioration of the toner or developer due to the elevation of temperature within the developing unit might degrade the image quality and fixing performance. So a toner which can also solve these problems together with the aforementioned problem has been desired.